Bodybuilders and others who are trying to reduce bodyfat are often advised to avoid late-night meals. The idea behind this advice is that you should consume the bulk of your calories when you're most active, such as before a workout, so the calories will be used as fuel and not
stored as bodyfat.
Past research disputed this idea, however. A few studies showed that when you eat is less important than your total weekly calorie intake, and scientists made the assumption that you don't get fat immediately but instead have a biochemical grace period of about 72 hours before excess calories merge into your fat cells. If you lower your calories over the next two days, or better yet, lower your calories and increase your physical activity, you can still nullify the excess calories. Even so, more recent evidence indicates that late-night meals may indeed promote greater fat deposition.
It has to do with a process called diet-induced thermogenesis, or DIT, which involves the way calories from meals are turned into heat, or energy. Generally speaking, when increased metabolism, in this case from the DIT, converts calories to energy, they can't be stored as bodyfat.
In a study reported in the April '93 issue of the American Journal of Clinical Nutrition, nine men ate morning, afternoon or late-evening snacks on three separate days over five weeks. After carefully monitoring these subjects, the researchers found a big difference in calories converted into energy from the morning and late-evening meals, specifically that the late-evening meals indicated a blunted DIT effect as compared to the morning meals. The afternoon meals, on the other hand, showed no big effect either way.
As to what promotes this rise in metabolism after a meal, scientists can't say for sure, They suspect it has something to do with the regular flow of hormones controlled by an internal body clock. Apparently, these hormones are more active earlier in the day. While eating a late-night snack won't really destroy a diet, the calories you consume at that time do have a greater chance of being converted into bodyfat and sticking with you.